drifting piston ring?

What can cause a piston ring to drift off the key? A friend of mine has an old kawi 350 triple, and he's running suzuki pistons, with the barrels bored out to match. Not sure what size the pistons are. His bike developed a "ticking" nose last night with only 80 something miles on the new top end. We found the the right piston had drifted off the key. We put it all back together again making sure everything was were it should be. When we kicked it over the nose was gone. But as soon as we started the bike it came back right away. Any thoughts???